Flagellates and ciliates as biocontrol agents for cyanobacterial blooms

Abstract

Harmful algal blooms (HABs), particularly cyanobacterial blooms, pose a significant global environmental threat. The existing physical and chemical control methods have significant limitations, highlighting the need for effective and environmentally safe alternatives. This review systematizes and analyses published and original data on the biotic interactions between flagellates, ciliates, and cyanobacteria and evaluates their potential as biocontrol agents. We demonstrate that mixotrophic flagellates of the genera Ochromonas and Poterioochromonas are the most promising candidates, as they combine resistance to toxic strains, the ability to actively graze on various cyanobacterial morphotypes, and the capacity to degrade secondary metabolites. Among ciliates, representatives of the genus Nassula show efficacy against filamentous forms. The efficacy of protist application is governed by multiple biotic and abiotic factors, including prey morphology, predator physiology, predator–prey cell ratios, environmental conditions, and interspecific interactions. Most studies are currently confined to laboratory experiments, and field validation is essential for technology scaling. Combined approaches that integrate biological control with physical or chemical methods are discussed, along with the potential use of machine learning for bloom forecasting and the optimization of inoculation parameters. We propose a conceptual design for a floating modular bioremediation station that combines biomass collection, ultrasonic colony disruption, protist-mediated grazing, and debris sedimentation as a potential solution. This work synthesizes current knowledge and outlines future research directions in protist-based biocontrol of cyanobacterial blooms.